1. Field of the Invention
The present invention relates to a switching power source generating direct current voltage.
2. Description of the Related Art
Switching power source of self-excited oscillation type (hereinafter, referred to as switching power source) is known power source, which converts a direct current voltage into constant voltage appropriate to be used in apparatuses. In this case, the direct current voltage is acquired by rectification and smoothing of an alternating-current voltage input from commercial power source. The switching power source is typically designed to produce maximum efficiency when loaded to a highest level required for an electronic device having a power source. Thus, lighter load on the switching power sources (i.e., light loaded state) decreases efficiency of the switching power sources. Consequently, electronic devices equipped with self-oscillating switching power sources are configured to enter a sleep mode to reduce power consumption while the devices are not operated.
In the sleep mode, the switching power sources do not continuously oscillate, but intermittently oscillate by repeating oscillation and stopping the oscillation at predetermined intervals (hereinafter, referred to as intermittent oscillation operation) to prevent loss in efficiency of switching operations while reducing power consumption. The operation is controlled by transmitting a signal that instructs to stop oscillation, from a secondary side to a primary side of the switching power source, and switching the mode of the routine sequence to the intermittent oscillation operation mode. Such intermittent oscillation operation is discussed in Japanese Patent Application Laid-Open No. 2003-284340 for example. In the intermittent oscillation operation, a switching device (e.g., field effect transistor: MOSFET) in a switching power source is kept turned on for longer time than normal, reducing the number of switching operations, which prevents decrease in operation efficiency. The intermittent oscillation operation can be controlled, for example, by measuring time using a timer incorporated in a microcomputer, and intermittently oscillating at intervals appropriate for a constant of a power source circuit and for a load to be connected
However, a problem described below occurs when the intermittent oscillation operation is performed. It was found that, in the intermittent oscillation operation, a noise is generated from a transformer. The noise has intensity peaks at a multiple of a frequency of which a cycle is interval between the oscillation operations. FIG. 9 is a graph illustrating an example of noise from a transformer, with the horizontal axis for frequency and the vertical axis for noise intensity, in the case where the intermittent oscillation operation is performed at a frequency of 10 KHz. FIG. 9 indicates the transformer generates noise showing the frequency characteristics in which the intensity peaks of the noise are distributed discretely. Such noise showing the frequency characteristics results from a frequency of noise generated by mechanical vibration of the transformer combined with high frequency components of the drive frequency of a switching device. A range of such noise is also known to be within human audibility, and is an unpleasant noise to human ears. In other words, during the intermittent oscillation operation, power consumption can be reduced without decreasing efficiency of switching power source, but electronic devices incorporated in the switching power source generate the unpleasant noise.